(Bloomberg) -- BT Group Plc’s biggest rival is ramping up the competitive threat to the former phone monopoly with a plan to connect all of its customers to fast fiber broadband.

Virgin Media O2 will upgrade its entire network of 15.5 million U.K. connections to fiber optics instead of cable by 2028, the venture said in a statement Thursday.

Until now, Liberty Global Plc’s Virgin Media used fiber mostly to fill gaps in its older cable infrastructure. Following a tie-up this year with Telefonica SA’s U.K. mobile operator O2, it now plans to replace the old cables entirely.

The announcement steps up the pressure on BT as it tries to convince investors that its own national fiber rollout will give it a competitive edge.

BT Chief Executive Officer Philip Jansen has focused on the upgrades to reverse years of disappointing results. The company said in May it will build an extra 5 million fiber optic connections by 2026 and explore funding them with a joint venture, opening up its infrastructure to external investors for the first time.

The government is leaning on the industry to accelerate the shift to fiber after the pandemic exposed the importance of Britain’s internet infrastructure. Download speeds over legacy cable technology tend to be much quicker than uploads. Upload bandwidth has become vital now more people are using their broadband for videoconferencing and online gaming.

Virgin Media O2 said it could achieve the upgrade at a low cost, with a quarter or less of greenfield construction, by threading fiber through its existing underground ducts.

Virgin Media’s network was patched together from several cable companies that merged after a boom and subsequent string of bankruptcies in the 1990s. The company chose to upgrade the legacy network using a process called DOCSIS. The move to fiber is an acknowledgment that this technology has its limits.

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